Conventionally, the focus of a motion picture camera lens is set manually to a distance where the plane of focus of the lens closely coincides with the location of a photographic subject. The camera lenses have focus distance calibrations that correspond to the distance from the image plane of the camera to the focus plane. Motion picture cameras may also have an electronic mechanism for delivering the focus distance setting through an electrical interface. The focus puller (e.g., a technician responsible for setting the focus of the camera lens) can adjust the camera lens to the subject distance by estimating the distance to a photographic subject and setting the focus distance of the lens to match the estimated distance, either directly or using electro-mechanical controls.
To estimate the focus setting for a static camera, the focus puller conventionally uses distance cues, such as reference marks made on the ground during rehearsal, or other objects whose known position relative to the camera can serve as distance references. Provided that the focus setting corresponds to the subject being within the depth of field of the lens, this depth being a specified range of distances in front of or behind the focus distance setting, the subject will appear acceptably sharp.
In many situations, the focus puller cannot achieve acceptably sharp focus using these conventional methods. For example, when the camera is moving during the process of shooting a scene, it is often not possible to use pre-set focus reference marks, as the path taken by the camera may not be predictable. In other situations, the depth of field of the lens is so shallow that the focus setting cannot be reliably estimated by the focus puller, even when reference marks are available.
In the situations described in the previous paragraph, the focus puller may use the image captured by the camera as displayed by a monitor to adjust the focus. However, the monitor can only show the degree to which the image appears to be in focus. If the image or subject is out of focus, the monitor cannot show the direction or the magnitude of any necessary focus setting change that will bring the subject coincident with the plane of focus, or within the depth of field of the lens.
There have been a number of challenges to focusing a camera (or image captured by the camera) using a video monitor. For example, once unclear or “buzzed” focus is observed, it is often too late to maintain or bring back focus, especially when subjects in the field of view of the camera are in motion. Sometimes it can be difficult to tell whether one must pull forward or backward to correct the focus. Peaking, or making the edges of objects within the image more visible (e.g., by increasing the contrast or color of the edges, or making the edges shimmer) does not provide much critical focusing information. Also, on many episodic TV series, there isn't sufficient room on set for the focus pullers, and they often work outside the set. On multiple camera shoots, there will be one monitor and one wireless focus unit for each camera and each assistant, thereby making demands for space for focus pullers even higher.
Distance measuring devices (DMD's) with single detection zones that are attached to the camera have been used both to autofocus the lens as well as to measure the distance between the camera and the subject. For example, U.S. Pat. No. 4,199,246 discloses an ultrasonic ranging system for autofocusing a camera. Subsequent ultrasonic DMD's have been attached to motion picture cameras to give focus pullers distance information for a single subject within the detection zone of the ultrasonic DMD.
The antenna pattern of an ultrasonic DMD includes a main lobe and attenuated side-lobes. Ultrasonic devices are most sensitive to targets within the main lobe. However, an off-axis target present in a side lobe may be detected and may cause incorrect focus. This characteristic can result in the ultrasonic DMD indicating erroneous distance especially if one or more targets are near the side-lobes. The single detection zone characteristic of ultrasonic DMD's limits the ability of the focus puller to discriminate between multiple targets located within the single detection zone of the ultrasonic DMD.
An additional disadvantage of devices that provide a digital display of the distance to a subject is that the user needs some time to interpret the difference between the distance display of the DMD and the present setting of the lens distance prior to changing the lens focus setting. The additional time that the user takes to interpret the digital information increases the difficulty of maintaining sharp focus in situations where the either the subject or camera is moving quickly.
Laser ranging (LR) devices have been attached to cameras for autofocus purposes as well as to provide distance measurements for manually focusing the camera lens. Such ranging devices detect targets within a small target detection area defined by the spot size of the collimated laser, and as a result, require precise targeting and tracking of the target. U.S. Pat. No. 8,363,152 B2 discloses such a device, where the alignment and tracking of the subject is carried out using a display device reproducing the shooting image, on which the object to be focused is selected, and the distance measurement instrument is aligned at the selected object and tracked.
Since LR devices typically have sensing beams with divergence angles much smaller than the typical horizontal or vertical angle of view of the camera lens, the operator of the device must precisely track the target. LR devices can measure and indicate the distance to a subject, and with appropriate correction for parallax, indicate the focus distance. Since the operator of the device is occupied with target tracking, the operator cannot manually alter the focus setting of the camera lens in accordance with the distance indicated by the LR device in a way that is consistent with the artistic intent of the scenario without difficulty. For this reason, LR devices may be well-suited for autofocusing, but disadvantageous for manual operation.
The disadvantage of auto-focus devices relative to manual focus control is that the rate at which the lens focus setting matches the subject distance is fixed by the settings of an automatic closed loop servo in auto-focus devices. Notwithstanding the possibility of providing a user with the facility to adjust the time response of the focus servo, the response must be preset in advance, precluding the matching of the time response to the movement of photographic subjects as it occurs. This disadvantage of autofocus devices is especially apparent when focus is changed between subjects. In such situations, the rate of change of the focus settings should be consistent with aesthetic considerations. More specifically, with reference to the movement of the subjects in certain contexts, settings or situations, a mechanical appearance to the focus change may be artistically undesirable.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.